The present invention relates to a method for inspecting photolithographic reticles used in the manufacture of semiconductor devices, and more particularly for inspecting the edges of reticle features. The invention has particular applicability for in-line inspection of reticles with submicron design features.
Current demands for high density and performance associated with ultra large scale integration require submicron features, increased transistor and circuit speeds and improved reliability. Such demands require formation of device features with high precision and uniformity, which in turn necessitates careful process monitoring.
One important process requiring careful inspection is photolithography, wherein masks, or xe2x80x9creticlesxe2x80x9d, are used to transfer circuitry features to semiconductor wafers. Typically, a series of such reticles are employed in a preset sequence. Each photolithographic reticle includes an intricate set of geometric features corresponding to the circuit components to be integrated onto the wafer, such as chrome features on a glass substrate. Each reticle in the series is used to transfer its corresponding features onto a photosensitive layer (i.e., a photoresist layer) which has been previously coated on a layer, such as a polysilicon or metal layer, formed on the silicon wafer. The transfer of the reticle features onto the photoresist layer is conventionally performed by an optical exposure tool such as a scanner or a stepper, which directs light or other radiation through the reticle to expose the photoresist. The photoresist is thereafter developed to form a photoresist mask, and the underlying polysilicon or metal layer is selectively etched in accordance with the mask to form features such as lines or gates on the wafer.
Fabrication of the reticle follows a set of predetermined design rules set by processing and design limitations. These design rules define, e.g., the space tolerance between devices and interconnecting lines and the width of the lines themselves, to ensure that the devices or lines do not overlap or interact with one another in undesirable ways. The design rule limitation is referred to as the xe2x80x9ccritical dimensionxe2x80x9d (CD), defined as the smallest width of a line or the smallest space between two lines permitted in the fabrication of the device. The design rule for most ultra large scale integration applications is on the order of a fraction of a micron.
As design rules shrink and process windows (i.e., the margins for error in processing) become smaller, inspection and measurement of reticle features is becoming increasingly important, since even small deviations of feature sizes from design dimensions may adversely affect the performance of the finished semiconductor device. For example, features on the surface of the reticle include straight lines that extend across the surface of the reticle for forming interconnection lines or gates. FIGS. 1A-1B illustrate some typical defects of straight-line features. FIG. 1A shows an oversized portion of a line manifested as a xe2x80x9cbumpxe2x80x9d on the line. The size of a non-defective feature is represented by the dotted line. FIG. 1B shows an undersized portion of a line manifested as a xe2x80x9cbitexe2x80x9d taken out of the line, and the size of a non-defective feature is represented by the dotted line.
Those skilled in the art recognize that a defect on the reticle, such as extra or missing chrome in straight lines as shown in FIGS. 1A-1B, may transfer onto the wafers during processing in a repeated manner, and therefore may significantly reduce the yield of the fabrication line. Therefore, it is of utmost importance to inspect the reticles and detect any defects thereupon. The inspection is generally performed by an optical system, such as the RT 8200 or ARIS-i reticle inspection system available from Applied Materials of Santa Clara, Calif. In the mask shop, i.e., where the masks and reticles are produced, the inspection system is used to scan the mask and compare the obtained image to the database used to create the mask. Differences between the image and the database are flagged as a suspect location.
More particularly, in typical prior art inspection schemes, the surface of the reticle is imaged, e.g., with a photomultiplier tube (PMT) or with a charge-coupled device (CCD) and the resulting image is an array of data elements called xe2x80x9cpixelsxe2x80x9d, each pixel being assigned a xe2x80x9cgray levelxe2x80x9d corresponding to its intensity. In other words, each pixel is assigned a gray level proportional to the light transmitted, reflected, or both, depending on the system""s design, by a portion of the reticle. For example, depending on the lighting and imaging technique used during scanning, a pixel located in the middle of an opaque feature will have a very high gray level, while a pixel in the space between features will have a low gray level, or vice versa. The pixels are typically analyzed one at a time and compared to pixels at the same respective location in a reference, such as a database, or the same location in a corresponding neighboring die, to determine the existence of defects. The gray levels of each of the pixels of the inspected reticle are also compared to the gray levels of their neighboring pixels to detect the edges of features for dimensional measuring purposes.
Disadvantageously, conventional reticle inspection tools cannot always accurately or reliably detect small or xe2x80x9clocalxe2x80x9d defects in straight-line features. Prior art inspection tools lack the necessary sensitivity because they are limited to performing their analysis one pixel at a time. There is a need for a simple, fast, cost-effective methodology for inspection of reticles that enables accurate detection of straight line defects.
An advantage of the present invention is the ability to reliably detect defects in straight line features without increasing inspection time.
According to the present invention, the foregoing and other advantages are achieved in part by a method of inspecting a target feature formed on a surface, the method comprising imaging the target feature to produce a matrix of pixels representative of the target feature, each pixel having a gray level and associated with an x and a y coordinate corresponding to its respective location on the surface. A small array of pixels, such as a 2xc3x972 array, that lays on an edge of the target feature is identified, a plane is calculated based on the x coordinate, the y coordinate and the gray level of each of the pixels in the array, and an angle xcex1 between a reference line (such as the positive x coordinate axis) and a line at the intersection of the calculated plane and a reference plane is calculated. Several other arrays adjacent to the first array and laying on the target feature edge are identified; e.g., about four other 2xc3x972 arrays, and their respective angles xcex1 are calculated. The angles xcex1 are then compared. If the xcex1""s are substantially equal, it is determined that the target feature edge is straight. On the other hand, if the xcex1""s are not substantially equal, it is determined that the target feature edge is curved. A corresponding reference feature image is analyzed in the same way as the target feature image, and a defect is determined to exist in the target feature when the target feature edge is curved and the reference feature edge is straight, indicating an unwanted bite or a bump on the target feature edge.
Another aspect of the present invention is an inspection tool for carrying out the steps of the above method.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description are to be regarded as illustrative in nature, and not as restrictive.